Iron analysis reagent formulation

ABSTRACT

A single reagent formulation for use in the spectrophotometric determination of iron, which is capable of carrying out the multiple functions required in the spectrophotometric determination, comprises a solution of thioglycolic acid, a buffer to maintain the pH of the solution at a predetermined value or range and a ferroin reagent present in an amount effective to form a colored complex with the iron in a test sample. Superior solubilizing of the iron in a test sample is achieved when ammonium hydroxide serves as the buffer.

United States Patent 1 Hach [ 51 Jan. 9, 1973 [54] IRON ANALYSIS REAGENTFORMULATION [75] Inventor: Clifford C. Hach, Ames, Iowa [73] Assignee:Hach Chemical Company, Ames,

Iowa

[22] Filed: July 20, 1970 [21] Appl. No.: 56,361

[52] U.S. Cl. ..23/230 R, 252/408 [51] Int. Cl. ..G01n 31/22, GOln 33/18[58] Field of Search ..23/230, 253; 252/408 [56] References Cited UNITEDSTATES PATENTS 3,506,404 4/1970 Evans et a1 ..23/230 R 3,537,822 11/1970OMalley et al. ....23/230 R 3,506,403 4/1970 Fryer et al. ..23/230 ROTHER PUBLICATIONS Furman, N. 11. ed., Standard Methods of ChemicalAnalysis, Vol. 1, p. 530 (1963) Hackhs Chemical Dictionary, p. 676(1965) Lindsay, F. K. et a1. Analytical Chemistry, Vol. 12, pp. 460-63(1940) lsbell, H. 6., Analytical Chemistry, Vol. 4, pp. 28486 (1932)Primary Examiner-Morris O. Wolk Assistant Examiner-12. M. ReeseAttorney-Wolfe, Hubbard, Leydig, Voit & Osann, Ltd.

57 ABSTRACT A single reagent formulation for use in thespectrophotometric determination of iron, which is capable of carryingout the multiple functions required in the spectrophotometricdetermination, comprises a solution of thioglycolic acid, a buffer tomaintain the pH of the solution at a predetermined value or range andaferroin reagent present in an amount effective to form a coloredcomplex with the iron in a test sample. Superior solubilizing of theiron in a test sample is achieved when ammonium hydroxide serves as thebuffer.

10 Claims, 2 Drawing Figures IRON ANALYSIS REAGENT FORMULATION Thisinvention relates to the quantitative analysis of iron and, moreparticularly, to a single reagent formulation characterized by itsability to carry out the multiple functions required in thespectrophotometric determination of iron.

It is desirable to quantitatively determine the presence of iron in awide variety of applications. For example, it is necessary tocontinuously analyze for iron in the boiler water in a steam electricpower generating plant because the iron oxide content of the boilerwater is an index of the rate of corrosion taking place in the boiler.

As is typically carried out at present, the quantitative determinationof iron involves solubilizing the solid iron compounds by adding to thesample an acid such as hydrochloric and then allowing time for the solidiron compounds to dissolve. Since almost all solid oxides of irondissolve slowly at room temperatures, the solubilizing step is generallycarried out by heating the sample containing the acid to boiling for asufficient period of time to dissolve the iron compounds. Some ironoxides, such as Fe O are extremely difficult to dissolve in diluteboiling acid solutions and many hours of boiling may be required toachieve solution. Accordingly, oxides of this type are generallydissolved by boiling with concentrated acid or mixtures of concentratedacids such as hydrochloric, nitric and perchloric acids.

The iron is then reduced to its ferrous form by adding to the samplesolution, a reducing agent such as hydroxylammonium chloride. This stepmay be, and generally is, carried out in connection with the initialaddition of hydrochloric acid.

A colorimetric reagent for iron is then added to the ferrous ironsolution and the solution buffered to adjust the acidity and pH of thesolution to the range in which the iron complex formed by the reagentand the ferrous ions is color responsive. The addition of thecolorimetric reagent and the buffering of the solution may be carriedout either together or separately.

While these determinations are of course carried out manually,continuous analyzing instruments are widely used. Such continuousanalysis of iron is typically carried out by mixing an incoming sampleat a specified rate with a minor proportion of solubilizing and reducingagents and then transporting to a chamber which is heated to effect thesolubilizing. A second reagent solution, including the colorimetricreagent and the buffer, is then added and the solution cooled to aboutroom temperature. The resulting solution is then transported to thecolorimetric cell and the determination made.

The necessity for employing two separate reagents complicates thedetermination of the iron as well as the continuous apparatus which isemployed. Additionally, the solubilizing step requires considerable timeand impedes timely analyses.

It is accordingly an object of the present invention to provide a singlereagent solution characterized by its ability to effect the multiplefunctions required in the spectrophotometric determination of iron. Amore specific object provides a single reagent solution capable ofsolubilizing the iron, reducing to its ferrous form and forming acolored complex.

A further object of the present invention is to provide a reagentsolution which is capable of increasing the dissolution rate of ironcompounds, especially refractory oxides of iron.

Another and more specific object lies in the provision of a reagentsolution which does not require the employment of heat, except fordifficulty dissolvable refractory iron oxides. A related object providesa reagent wherein a single component provides both the buffering and anincrease in the rate of dissolving of the iron compounds.

Other objects and advantages will become apparent as the followingdescription proceeds taken in conjunction with the following drawings inwhich:

FIG. 1 is a graph of percent iron in a test sample that has beendissolved and colorimetrically determined plotted against time for twodifferent reagent formulations which were added to the test samples, oneof which embodies the present invention; and

FIG. 2 is a graph of percent iron in a test sample that has beendissolved and colorimetrically determined plotted against the reagentformulation used to illustrate the optimum formulation for thecomposition of the present invention.

While the invention is susceptible of various modifications andalternative forms, certain specific embodiments thereof have been shownby way of example and will be described in detail herein. It should beunderstood, however, that it is not intended to limit the invention tothe particular forms disclosed, but, on the contrary, the intention isto cover all modifications, equivalents and alternatives falling withinthe spirit and scope of the invention.

Briefly, and in accordance with the present invention, there is provideda single reagent solution which is capable of carrying out the functionswhich heretofore has required as many as four reagents, and never lessthan two separate reagents, and in more efficient manner. Thus, thereagent of the present invention includes a solution of thioglycolicacid, a buffer for the acid to maintain the pH within a predeterminedrange and a ferroin reagent present in an amount effective to form acolored complex with the iron in the sample being tested at the pHlevel. In the preferred embodiment, the buffer employed is ammoniumhydroxide, which, in addition to its buffering, increases the rate ofdissolving of the iron oxides.

To carry out an iron determination, about one volume of the reagentshould be added to about 25 to 50 volumes of the test sample. Thereagent solution can comprise from about 55 to about volumes of 98percent thioglycolic acid and from about 20 to about 45 volumes of abuffer, which is present in sufficient quantity to maintain a pH withinthe range of from about 3 to about 7, and a quantity of the ferroinreagent effective to form a colored complex with the ferrous iron in thesample. Any of the conventionally used buffers may be employed.Representative examples include various amines such as ethylenediamine,diethanolamine and amino methyl propanol and ammonium hydroxide.

With respect to the ferroin reagent, any conventional material can beemployed. Representative examples of known ferroin reagents include1,10-phenanthroline; 4,7-diphenyl-l,IO-phenanthroline and 2,4,6-tris(2-pyridyl)-l,3,5triazine. Others include 2,2 bypyridine and 2,2',2"tripyridine.

A particularly advantageous ferroin reagent is 3-(2- pyridyl)-,6-bis(phenylsulfonic acid )-1 ,2,4-triazine (the principal product hasthe sulfonic acid groups located at the 4-position but minor amounts maybe present having the acid groups located at other positions) or any ofits soluble salts such as the disodium, dipotassium and diammoniumsalts, the corresponding mono salts, or mixtures thereof. These ferroinreagents and their methods of preparation are disclosed in the copendingapplication of Stookey, entitled: Ferroin Reagent and Methods of MakingSame, Ser. No. 49,579, filed Jan. 24, 1970. Such reagents are soluble inwater, low in cost, have a minimum of interference and a sensitivitywhich exceeds that of other commercially available reagents for iron. Acolored complex with ferrous iron is formed at a pH of from about 4 to9.

The ferroin reagent should be present in the reagent solution in anamount of from about 2 to about grams/liter of the reagent solution.Functionally, the reagent should be present in an amount sufficient tocomplex together with the quantity of iron present in the test sample.

The preferred embodiment of the present invention comprises a reagentsolution having two volumes of 98 percent thioglycolic acid, one volumeof concentrated ammonium hydroxide and 5 grams/liter of the sodium saltof 3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)- 1,2,4-triazine. Thissolution has an iron rust and oxide dissolving ability many timesgreater than the dilute hydrochloric acid which is traditionally used.It is also capable of developing an accurate iron test in water analysisfor ordinary soluble iron and ordinary. iron rust without even anecessity for heating the sample to which the reagent is added.

The following examples are illustrative of the present invention but arenot in limitation thereof.

EXAMPLE 1 To illustrate the superior iron dissolving properties of thereagent of the subject invention, a comparison was made with asolubilizing agent which is widely used.

A 50 ml. sample of water containing suspended Fe O (prepared by burningsteel wool, pouring the resulting mo, powder into water, allowing thecoarse particles to settle and then separating and using the water withthe Fe O suspended therein) was treated with 1 ml. of 1/1 concentratedHCl plus 0.1 grams of hydroxylammonium chloride reducing agent broughtto boil and held at the boiling point of the solution for an extendedperiod of time.

Another 50 ml. sample of water containing suspended F630 and prepared ashereinbefore described was treated with a reagent solution comprising0.66 ml. 98 percent thioglycolic acid, 0.33 ml. concentrated ammoniumhydroxide and 0.1 grams of the sodium salt of3-(2-pyridyl)-5,6,bis(phenylsulfonic acid)-l,2,4-triazine. It was alsobrought to a boil and held at its boiling point for an extended periodof time.

Portions of the first sample hereinbefore identified were taken atvarious times and a reagent formulation comprising 1 ml. of ammoniumhydroxide ammonium acetate (taken from a 1 liter solution consisting of500 ml. of concentrated NH OH and 250 grams of ammonium acetate) with0.005 gms. of a ferroin reagent consisting of l,l0(ortho)phenanthrolineadded thereto.

The amount of the iron dissolved in the sample was then colorimetricallydetermined. The results are shown as curve A of FIG. 1.

Portions of the other sample were also taken at various times and theamount of iron dissolved at that time measured by the same colorimetrictechniques. Curve B of FIG. 1 illustrates these results.

EXAMPLE 2 This example illustrates the increased water-dissolvingproperties when ammonium hydroxide serves as the buffering agent for thethioglycolic acid.

A series of 50 ml. samples of water containing suspended Fe o (formed asin Example 1) were treated with 1 ml. of a reagent solution. Each samplewas treated with a reagent containing 98 percent thioglycolic acid and adifferent amount of concentrated ammonium hydroxide together with 0.010grams of the sodium salt of 3-(2-pyridyl)-5,6,bis(phenylsulfonicacid)-1,2,4-triazine. The various solutions were brought to boiling andthe parts per million of the ferrous iron were determinedcolorimetrically after the solution had been allowed to boil for 10minutes. The results are shown in FIG. 2.

EXAMPLE 3 To further illustrate the superior iron dissolving propertiesof the reagent of the subject invention set forth in Example 1, anothercomparison was made with the solubilizing agent used therein.

A 250 ml. sample of water containing suspended Fe O (prepared by burningsteel wool, pouring the resulting Fe 0 powder into water, allowing thecoarse particles to settle and then separating and using the water withthe Fe O suspended therein) was treated with 5 ml. of ill concentratedHCl plus 0.5 grams of hydroxylammonium chloride reducing agent and heldat the boiling point of the solution for an extend period of time.

Another 250 ml. of the same sample was treated with 5 ml. of a solutioncomprising two parts by volume 98 percent thioglycolic acid and one partby volume of concentrated ammonium hydroxide. It was also held at itsboiling point for an extended period of time.

The amount of iron dissolved and reduced to the ferrous ion wascolorimetrically measured by the use of a ferroin reagent consisting of1,10-phenanthroline at various times. The results are shown in Table 1:

dissolving properties when ammonium hydroxide serves as the bufferingagent for the thioglycolic acid which was shown in Example 2.

A series of 250 ml. samples of water containing suspended Fe o (formedas in Example 1) were treated with 5 ml. of a reagent solution. Eachsample was treated with a reagent containing 98 percent thioglycolicacid and a different amount of concentrated ammonium hydroxide. Thevarious solutions were brought to boiling and the parts per million ofthe ferrous ion was determined as in Example 3 after the solution hadbeen allowed to boil for minutes.

The results are shown in Table 2:

TABLE 2 Thioglycolic Acid 96 NH OH ppm Fe dissolved 100 0 0.83 ppm foundThus, as has been seen, the present invention provides a single reagentsolution which is capable of solubilizing the iron in a test sample,reducing the iron to its ferrous form, buffering the sample solution toa predetermined pH and forming a color complex with the iron present.The reagent solution has an iron rust and oxide dissolving ability manytimes greater than dilute hydrochloric acid which is traditionally usedand is capable of developing an accurate iron test in water analysis forordinary soluble iron and ordinary iron rust without even a need forheating the sample at all. In addition, the reaction mixture can be usedover extended periods without any deterioration. While the reagentsolution of the present invention may, of course, be employed forlaboratory analyses, if finds particularly advantageous use whenemployed in connection with various commercial continuous analyzers.

I claim as my invention:

1. A method of quantitatively determining the presence of iron in asample solution which comprises adding to a sample solution a singlereagent formulation consisting essentially of thioglycolic acid presentin an amount sufficient to dissolve the iron compounds in the samplesolution and reduce the iron to its ferrous form, a buffer to maintain asolution of the reagent and sample solution at a predetermined pH and aferroin reagent present in an amount effective to form a colored complexwith the ferrous iron in the reagent sample solution at the pH levelallowing the colored complex to form and thereafter colorimetricallydetermining the amount of iron in the sample.

2. The method of claim 1 wherein, by volume, one part of the reagentformulation is combined with from about 25 to about 50 parts of thesample.

3. A reagent formulation for use in the spectrophotometric analysis ofiron in a test sample solution which consists essentially ofthioglycolic acid present in an amount sufficient to dissolve the ironcompounds in the test sample solution and reduce the iron to its ferrousform, a buffer to maintain a solution of the reagent and test samplesolution at a predetermined pH and a ferroin reagent present in anamount effective to form a colored complex with the ferrous iron in thereagenttest sample solution at the pH level, said reagent formulationbeing capable of addition to the test sample solution as a singlereagent to develop an accurate analysis for iron.

4. The reagent formulation of claim 3 wherein, by

volume, the thioglycolic acid is present in an amount of from about 55to about parts and the buffer is present in an amount of from about 20to about 45 parts.

5. The reagent formulation of claim 3 wherein the ferroin reagent ispresent in an amount of from about 2 to about 10 gms./liter of thereagent solution.

6. The reagent of claim 3 wherein the buffer is ammonium hydroxide.

7. The reagent formulation of claim 6 wherein, by volume, thethioglycolic acid is present in an amount of from about 55 to about 80parts and the ammonium hydroxide buffer is present in an amount of fromabout 20 to about 45 parts.

8. The reagent formulation of claim 3 wherein the ferroin reagent is asalt of 3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)-l,2,4-triazine, thesalt being a member selected from the group consisting of monosodium,disodium, monopotassium, dipotassium, monoammonium, diammonium andmixtures thereof.

9. The reagent formulation of claim 8 wherein the salt comprisesdisodium.

10. The reagent formulation of claim 8 wherein the buffer is ammoniumhydroxide.

2. The method of claim 1 wherein, by volume, one part of the reagentformulation is combined with from about 25 to about 50 parts of thesample.
 3. A reagent formulation for use in the spectrophotometricanalysis of iron in a test sample solution which consists essentially ofthioglycolic acid present in an amount sufficient to dissolve the ironcompounds in the test sample solution and reduce the iron to its ferrousform, a buffer to maintain a solution of the reagent and test samplesolution at a predetermined pH and a ferroin reagent present in anamount effective to form a colored complex with the ferrous iron in thereagent-test sample solution at the pH level, said reagent formulationbeing capable of addition to the test sample solution as a singlereagent to develop an accurate analysis for iron.
 4. The reagentformulation of claim 3 wherein, by volume, the thioglycolic acid ispresent in an amount of from about 55 to about 80 parts and the bufferis present in an amount of from about 20 to about 45 parts.
 5. Thereagent formulation of claim 3 wherein the ferroin reagent is present inan amount of from about 2 to about 10 gms./liter of the reagentsolution.
 6. The reagent of claim 3 wherein the buffer is ammoniumhydroxide.
 7. The reagent formulation of claim 6 wherein, by volume, thethioglycolic acid is present in an amount of from about 55 to about 80parts and the ammonium hydroxide buffer is present in an amount of fromabout 20 to about 45 parts.
 8. The reagent formulation of claim 3wherein the ferroin reagent is a salt of3-(2-pyridyl)-5,6-bis(phenylsulfonic acid)-1,2,4-triazine, the saltbeing a member selected from the group consisting of monosodium,disodium, monopotassium, dipotassium, monoammonium, diammonium andmixtures thereof.
 9. The reagent formulation of claim 8 wherein the saltcomprises disodium.
 10. The reagent formulation of claim 8 wherein thebuffer is ammonium hydroxide.